One of the inventors herein is also the inventor of several prior patents related to the improved enhancement of a patient's organ undergoing a computed tomographic scan by controlling the injection of a contrast medium in accordance with a predicted enhancement. These patents include U.S. Pat. Nos. 5,583,902 and 5,687,208, the disclosures of which are incorporated herein by reference. Still other patents have been issued relating to contrast injectors and their use in order to obtain specific enhancement levels. These include U.S. Pat. Nos. 5,827,219; 5,840,026; 5,383,858; 5,662,612; 5,681,286; 5,456,676; and 5,300,031; the disclosures of which are incorporated herein by reference. Still other patents which have been issued and which relate to the field of contrast injectors and their use include U.S. Pat. Nos. 4,006,736; 5,868,710; 4,854,324; 4,210,138; 4,812,724; 5,865,744; 5,279,569; 5,865,805; 4,213,454; 4,695,271; 5,322,511; 5,269,762; and 5,851,184; the disclosures of which are also incorporated herein by reference.
While the use of contrast injectors for injecting a patient with a contrast agent in order to enhance a tissue or organ for CT scanning has been done for years, the first two patents mentioned above, i.e. the '902 and the '208 patents, represent one of if not the earliest attempt to scientifically solve the problem of computing an expected enhancement based upon a patient's physical parameters, assuming a given injection protocol. The inventors work in these prior patents was directed at solving this problem for a patient's organs, using complex differential equations and their solutions to help answer how a human body functions in processing the contrast agent, and then calculating a window of enhancement for which the threshold of desired enhancement is exceeded for a successful scan, assuming a typical injection protocol. As part of the '208 patent, CT angiography was described and its special problems in obtaining high quality scans of blood vessels.
CT angiography (CTA) has been widely accepted, in some cases preferred over conventional angiography, to evaluate the anatomy of major blood vessels such as the aorta and pulmonary artery. In the prior art, the vessels are scanned using a thin-collimation spiral CT technique, while a bolus of contrast medium is injected at a high injection rate (3-5 mL/s) to achieve a high degree of vascular contrast enhancement. Typically, contrast medium is injected at a constant injection rate, i.e. a uniphasic injection protocol is used. This injection scheme results in a steadily rising vascular contrast enhancement profile with a single peak of enhancement occurring shortly after the completion of the injection, as shown by the data collected from a porcine experiment as shown in FIG. 1a. Consequently, vascular enhancement tends to be non-uniform during image acquisition.
Uniform vascular enhancement through the entire period of image acquisition is highly desirable for the purpose of image processing and display, in which 3D postprocessing is frequently based on a threshold CT attenuation value. In addition, it is expected that uniform enhancement would contribute to an optimized usage of contrast medium. In other words, for a given volume of contrast medium, a uniform contrast enhancement whose magnitude is lower than that of a peak enhancement generated by a uniphasic injection would provide a longer temporal window of adequate vascular enhancement than the uniphasic injection presently used in the prior art, thereby resulting in a longer optimal scanning interval. Alternately, it is expected that a smaller volume of contrast medium would be needed to provide a uniform vascular enhancement for the same scanning duration as that achieved by using a uniphasic injection protocol.
In addition to a uniphasic injection protocol, a biphasic injection protocol is sometimes used as well in the prior art. A typical biphasic injection protocol consists of two phases: a short rapid-injection phase, followed by a longer slow-injection phase. A biphasic injection protocol yields more prolonged enhancement than a uniphasic injection protocol, but it generates two enhancement peaks with a valley of enhancement in between. Data collected from another porcine experiment as shown in FIG. 1b supports this conclusion. Each peak occurs shortly after the completion of each injection phase, as might be expected given the results from the first porcine experiment. Although the biphasic might be considered by one less sophisticated as a step in the right direction, it actually increases the complexity of the problem of first of all achieving a level of enhancement which reliably exceeds a threshold and then maintaining an enhancement level above that threshold for a time period that will be adequate to collect the image. As the prior art has little to teach with respect to solving this problem, other than the inventors own work which has not yet focused on the injection protocol aspect of the problem, it would seem that unguided use of a biphasic injection protocol would perhaps increase the amount of contrast agent injected needed to reliably achieve a successful scan over that of a simple uniphasic injection protocol whose enhancement is easier to predict.
The present invention carries the inventor's prior work further by focussing on the injection protocol for CT angiography of the vascular system of a patient, and more particularly by computing an optimum solution of a specific contrast injection protocol for optimizing both the level of enhancement as well as the temporal duration of the enhancement, and an injector to achieve such injection protocol. A byproduct of this invention is as before, the ability to minimize the amount of contrast agent needed to be injected into a patient in order to reliably obtain a successful scan. This is important not only from a cost standpoint as the contrast agent can be expensive, but also from a health standpoint for the patient. The smaller the amount of contrast agent injected into a patient's body the less risk of harmful side effects.
More particularly, the inventors herein have succeeded in developing a contrast injector and a contrast injector protocol for implementation in the contrast injector which optimizes the use of the contrast agent to reliably achieve an enhancement in excess of a preselected threshold and to maintain that "excess" level of enhancement for a temporal duration that is near optimal given the amount of contrast agent used. The contrast injection protocol comprises a ramped, or multiphasic, or exponentially decaying, or steadily decreasing injection rate. An ideal solution is provided by the solution of differential equations describing a simplified compartment model in lieu of the more complex whole body model taught in one of the inventors prior patents mentioned above. This solution renders an exponentially decaying rate of injection having a particular decay coefficient. However, it is contemplated that in the real world, this exponentially decaying injection rate could be approximated by a linearly decay, or ramped decay, or even multi-step decay and yet yield acceptable results in accordance with the teachings of the present invention. Indeed, in the real world, something less than a true exponentially decaying injection rate must necessarily be the physical limit of a contrast injector, even the computer programmable one taught herein as part of the invention. Perhaps even more so as the computer programmable injector uses digital control which in actuality is a series of relatively small steps in changing the injection rate. Still another factor to consider and which also minimizes the precision that might be thought necessary is the varying physiological response between different patients to the injected contrast agent. As such, mathematical precision is not considered as a limit to the present invention.
The particular exponential decay coefficient calculated is proportional to the cardiac output per body weight of the patient. Experimental data with pigs suggests that a decay coefficient of approximately 0.01 would be appropriate for humans. In order to render the injector easier to implement for a typical attending professional, the cardiac output of the patient could be assumed in advance as average and thus no patient specific input need be made in order to achieve an acceptable scan. The experimental data suggested that the decay coefficient designed to generate a uniform enhancement for normal cardiac output resulted in a more dome-shaped enhancement with an increased magnitude for a subject with impaired cardiac output, demonstrating the effect of cardiac output on contrast enhancement. In theory, albeit difficult in the real world, if the degree of cardiac output reduction is known, the exact same uniform vascular enhancement can be reproduced for patients with reduced cardiac output. This can be achieved by lowering the initial injection rate and decay coefficient calculated for patients with normal cardiac output, proportional to the reduction in cardiac output. However, it is apparent that a multiphasic injection protocol designed to achieve a certain level of vascular enhancement in patients with normal cardiac output will not result in overestimation of contrast medium enhancement in patients with reduced cardiac output. The term multiphasic is intended to refer to the injection protocol which is the subject of the present invention. It is to be distinguished from the simple uniphasic or biphasic protocols of the prior art, and represents a protocol which is variable over time in a decreasing fashion whether continuously or discontinuously, ramped, linear, curvilinear, or intermittently.
The duration of aortic enhancement can be prolonged either by increasing the volume of contrast medium for a given initial injection rate or by injecting slowly at a lower initial rate for a given contrast medium volume. With a uniphasic injection, peak magnitude of aortic enhancement depends on three injection factors, i.e. the concentration, injection rate, and total volume of contrast medium. With a multiphasic injection protocol, however, the peak magnitude can be independent of the total volume of contrast medium, provided that the volume is not too small to reach an initial upslope enhancement to a plateau or threshold level. Thus, multiphasic injection is advantageous over uniphasic injection when a prolonged duration is desired, while keeping contrast enhancement from rising, by increasing the volume of contrast medium. Using the teachings of the present invention, this multiphasic injection protocol represents the ideal protocol to reliably achieve a uniform enhancement in excess of a threshold value for a desired temporal window as necessary for a vascular scan.
While the principal advantages and features of the present invention have been briefly explained, a fuller understanding of the invention may be gained by referring to the drawings and the detailed description of the preferred embodiment which follows.